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“Dearest Ladies and Gentlemen, I would like to introduce our little girl Westlyn Reign Brolin (Bean),” he wrote of a video showing the newborn sound asleep. “Mama Kathryn was stellar during this miracle birth and Bean is a flawless gem through and through. We are blessed to the core and appreciate so much the love and support during this pregnancy from ALL who have partaken in our journey, directly and/or indirectly.”

“#beanlove #dogtown,” he added.

Brolin, 50, has two other children — Eden, 24, and Trevor, 30 — from his first marriage to actress Alice Adair. His second marriage was to actress Diane Lane, with whom he had no children. The latter pair stayed married until 2013.

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Boyd, 31, is the creator of Midheaven Denim, a line of jeans for taller women. The 5’11” new mom is also a model and was Brolin’s personal assistant before the two became engaged in 2015. The couple married in 2016.

The Avengers: Infinity War actor announced the couple’s pregnancy on Instagram in May.

“There’s a new sheriff in town and she’s no bigger than a sweet potato. Hang on to your hats,” Brolin captioned the photo slideshow of Boyd showing off her baby bump.

RELATED VIDEO: Baby Girl on the Way for Josh Brolin and Kathryn Boyd

The new baby girl’s step-grandmother is icon Barbra Streisand. The Funny Girl star, 76, has been married to fellow actor James Brolin, 78, since 1998.

In June, Brolin joked that Streisand is a “typical Jewish grandmother,” telling PEOPLE and Entertainment Weekly’s Editorial Director Jess Cagle that Streisand got emotional over a special gift Boyd gave her.

“My wife got her a bracelet that said ‘Grandma’ on it and she started crying,” the Sicario: Day of the Soldado actor said. “She is one of those … it’s annoying but in a really great way, [with] emails every day and like, ‘Hey, I thought of this name, what do you think of that? … When can I come over and feel the baby?’ “